A press-formed part having a hat-shaped cross section and an external shape being curved in its lengthwise direction is used as a skeleton structure member of a body shell of automobile, such as a side member (a front side member and a rear side member). This press-formed part is a critical part for ensuring crash performance of an automobile, and is required higher strength for enhancing collision safety and promoting weight a reduction of the body. Conventionally, this press-formed part has been produced by applying a press-forming method based on the draw-forming of a metal material sheet.
FIG. 18A is a perspective view to show an example of a curved part having a hat-shaped cross section (hereafter, simply referred to as a “curved part”) 10, FIG. 18B is a top view of the curved part 10, FIG. 18C is a side view of the curved part 10, FIG. 18D is a front view of the curved part 10 seen from one end thereof, and further FIG. 18E is a perspective view to explain a convex curved portion 11a and a concave curved portion 11b of the curved part 10. Further, FIG. 19A is a cross-sectional view to schematically show a drawing machine for producing the curved part 10 by prior art, FIG. 19B is a cross-sectional view to show a stage in which a metal material sheet 201 is interposed and constrained between a die 202 and a blank holder 203, and between the die 202 and a punch 204, FIG. 19C is a cross-sectional view to show a stage in which the punch 204 is pushed inward from the stage shown in FIG. 19B, and FIG. 19D is a cross-sectional view to show a stage in which the punch 204 is further pushed inward from the stage shown in FIG. 19C to be fully pushed inward with respect to the die 202. Further, FIG. 20A is a perspective view to show the metal material sheet 201, and FIG. 20B is a perspective view to show a drawn panel 301 which is produced by prior art shown in FIGS. 19A to 19D.
For example, when producing the curved part 10 shown in FIGS. 18A to 18E, first, as shown in FIG. 19A, the metal material sheet 201 shown in FIG. 20A is placed between the blank holder 203 and the die 202, and between the punch 204 and the die 202. Next, as shown in FIG. 19B, the periphery of the metal material sheet 201 is interposed and pressurized by the die 202 and the blank holder 203. Next, as shown in FIGS. 19C and 19D, the central portion of the metal material sheet 201 is caused to bulge by the punch 204, thereby draw-forming the metal material sheet 201 into a desired shape. Since, in this process of draw-forming, the periphery of the metal material sheet 201 is held by the die 202 and the blank holder 203 as shown in FIG. 19C, the inner portion of the metal material sheet 201 is formed by the punch 204 while being subjected to tension. The metal material sheet 201 is caused to flow into the die 202 with its periphery being constrained to some degree by the die 202 and the blank holder 203, thus being formed into the drawn panel 301 having the shape shown in FIG. 20B. The curved part 10 shown in FIGS. 18A to 18E is produced by cutting off an unwanted part of the periphery of the drawn panel 301.
FIG. 21 is a perspective view to show regions of the drawn panel 301 shown in FIG. 20B, where cracks and wrinkles are likely to occur.
When producing the curved part 10 by a press-forming method based on draw-forming, as shown in FIG. 21, wrinkles attributable to a surplus of the material are likely to occur in a top plate 301a of a concave curved portion 311b and a flange 301b of a convex curved portion 311a of the drawn panel 301 at a stage in which the metal material sheet is formed into the drawn panel 301. In order to suppress the occurrence of wrinkles, it is effective in general to prevent the metal material sheet 201 from excessively flowing into the die 202 by enhancing the constraint of the periphery of the metal material sheet 201 in the forming process by means of increasing the pressurizing force of the blank holder 203 or adding a draw bead on the blank holder 203.
However, when the metal material sheet 201 is prevented from flowing into the die 202 by those means, the sheet thickness will be excessively decreased in the top sheet 301c of the convex curved portion 311a, the flange 301d of the concave curved portion 311b, and an end portion 301e in the lengthwise direction of the drawn panel 301 so that a crack will occur in these areas 301c, 301d, and 301e when the metal material sheet 201 is a material having low extensibility.
For that reason, to produce a curved part 10, which is represented by, for example, a side member, by a press-forming method based on draw-forming without causing cracks and wrinkles, it becomes difficult to use a high-strength material (for example, a high tensile strength steel sheet) having low extensibility as the metal material sheet 201 so that a low strength material having high extensibility has to be used. For this reason, the sheet thickness of the metal material sheet 201 has to be increased to ensure the strength required for the curved part 10, and thus the weight reduction of a skeleton structure member of a body shell is impaired.
Further, when producing the curved part 10 by a press-forming method based on draw-forming, since an unwanted part which exists in the outer periphery of the drawn panel 301 is to be cut off and discarded, the yield of the material will also decrease.
It is noted that Patent Literatures 1 to 4 propose a technique to bend-forming a part having a cross section, which is simple and uniform in the lengthwise direction, such as a hat-shaped cross section and a Z-shape cross section. However, Patent Literatures 1 to 4 do not disclose a method for producing a part having a complicated shape such as that of the curved part 10.